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Unanswered: slow queries precisely every 30 seconds!

Hi, I hope someone may be able to help with a strange problem I'm having...

Precisely every 30 seconds, 1 or more queries end up in my slow query log because they take 3 or more seconds to run. These are often simple primary key selects which should be instant, and there's no one particular query/table which is the culprit.

The preciseness (they are timestamped exactly every 30 seconds, and there are never slow queries in between times) leads me to believe there's some sort of background process kicking in.

I have no cron jobs on this machine, and there are no application use cases which would fire a certain query every 30 seconds. This is a fairly busy server (several hundred qps) running OpenSolaris, which otherwise operates perfectly, apart from this small hiccup every 30 seconds!

Can anyone suggest how I may start to trace what's causing this? Perhaps someone knows of a mysql internal processes which fires every 30 seconds by default?